Investigators have seized hundreds of fake digital thermometers amid fears inaccurate readings could pose a serious threat to children with meningitis.

More than 400 cheap imitations were collected following raids in Harrow and Oxford by the Medicines and Healthcare products Regulatory Agency (MHRA).

Bogus digital thermometers are sold online for as little as 99p and can seriously endanger health, particularly among those with potentially fatal illnesses.

The MHRA launched an investigation after the parents of a young leukaemia sufferer were forced to rush their child to hospital for urgent care despite an apparently normal temperature reading.

The child had a high temperature that was not picked up by the thermometer they bought on the internet.

The raids formed part of a nationwide operation to prevent unapproved medical devices being sold on the internet. Many appear on auction site eBay and are priced below market value.

Dr Nicola Lennard, deputy clinical director at the MHRA, said: "Inaccurate readings from cheap, fake thermometers could result in a delay to a child getting the medical treatment they need and it is vital that people do not buy or use cheap, unapproved medical devices.

"The MHRA is working with internet sites to ensure that fake medical devices are not sold to people and we urge the public to report faulty medical devices."

The MHRA has seized a number of other fake medical devices from locations around the UK as part of its investigation.

These include seven Kiddicare cool pads designed to help reduce children's temperatures, and three counterfeit Slendertone devices used to tone stomach muscles.

The agency is working with eBay to prevent the sale of unapproved devices and is looking into how such products were supplied on the UK market.

The fake thermometers have no recognised brand name and often lack the requisite safety markings, warnings and instructions for use.

They can be identified by missing details such as an absence of a batch identification number, no CE mark to prove the item meets European standards and no four-digit identification number to show it has been through the appropriate safety assessment. Many are also sold without an instruction leaflet.

Anyone with a faulty device is urged to contact the MHRA's adverse incident hotline on 020 3080 6080 or go to the website at mhra.gov.uk.